With a self-imposed deadline of March 31 to reach a framework deal on Iran's nuclear program looming, representatives of the countries involved in the negotiations were engaged in frantic last-minute diplomacy.
U.S. President Barack Obama and German Chancellor Angela Merkel spoke by phone on March 27.
A White House statement said the two reaffirmed their commitment to a "long-term comprehensive deal that fully and verifiably ensures that Iran's nuclear program is exclusively peaceful."
The statement said "Iran must make the decisions necessary to resolve several remaining issues."
U.S. Secretary of State John Kerry spoke by phone with Russian Foreign Minister Sergei Lavrov and the two reportedly agreed to a "possible" meeting of the 5+1 group -- Britain, China, France, Russia, the United States, and Germany -- with Iran on March 29.
Reports from Brussels said EU foreign policy chief Federica Mogherini would join the talks in Lausanne, Switzerland, on March 29.
French Foreign Minister Laurent Fabius has said he would join Kerry and Iranian Foreign Minister Javad Zarif in Lausanne on March 28.